540 ALVERSON BLVD 2017-06-20 - Laserfiche WebLink

ameO Conceptual repair ideas for improving stability of the landslide scarp have focused on constructing a reinforced soil slope or stepped reinforced soil retaining walls. The reinforcement would be provided by geogrids. Site Conditions The following sections summarize the site conditions and stability of the slope. Surface Conditions AMEC visited the site on February 8 and March 5, 2013, to assess slope and landslide conditions at the site. The residential lot is relatively level and landscaped with a grass lawn surrounded by planters with shrubs and mulch. The back property line, at the northwest end of the lot, is very close to the top of the steep slope that forms the Alverson-Maulsby Bluff. The mulch planting beds extend to the top of the slope. Below the top of the slope, the vegetation consists mostly of blackberries and a few native shrubs and deciduous trees. The slope descends from the property line at 50 to 70 percent for a drop of 25 to 30 feet to an old road grade that is relatively level for about 14 feet in width. The slope continues below the old road at approximately the same steepness. The landslide occurred at a section of the slope that appeared slightly steeper than the surrounding slopes. The width of the rupture at the crown was 35 feet, which includes a tension crack that extends northward from the main slide. The obvious surface of rupture, in which the depleted mass flowed down the slope, is approximately 16 feet wide at the main scarp. The toe of the surface of rupture was 13 feet wide, although the adjacent 10 feet of soil to the north (right flank) may have displaced a small distance. The length of the surface of rupture was approximately 30 feet. The thickness of the slide mass was estimated to be 5 feet. Slide debris flowed down to the old road and down the slope below the old road. Figures 1 and 2 display the extent of the landslide. The landslide appears to have occurred within weathered surficial soils situated on a steep slope. The slide probably initiated as a transitional slide, but quickly turned into a debris flow. It appears similar to other slides that have occurred on the Alverson-Maulsby Bluff, including recent slides below the American Legion Memorial Park, further north along the Bluff. Subsurface Conditions Subsurface conditions were investigated by reviewing to the Geologic Map of the Marysville Quadrangle, Snohomish County, Washington by James P. Minard (USGS, 1985) and reviewing the boring log B-1 drilled at the site for a geotechnical engineering evaluation completed by Nelson Geotechnical Associates (February 2008), and our site reconnaissance of the soils exposed in the AMEC Project No.3-917-17245-A 2 W:\_Projects\17000s\17245 Alverson-Maulsby Bluff Property LLC\17245-A\Geotech Report\Finley Geotech Report 130722.docx